The present invention relates to an improved drying apparatus and method for removing volatile liquid from a liquid bearing web of material, which apparatus and method may find particular application in drying a web moving web of paper or like material.
In conventional paper manufacturing processes, a slurry of fibers and water in a head box is permitted to flow onto a support of woven wire material, known as a Fourdrinier wire belt which is moved beneath the head box at a uniform speed. Water drains through the Fourdrinier belt, thus leaving a thin layer of intermeshed fibers. Drainage of the water from the fibers may be assisted by suction boxes beneath the Fourdrinier belt. The resulting web may be transferred onto a felt belt for further drying. Water may also be removed from the web by feeding it between a series of press rollers and between felt covered rolls. The paper web may then pass around a series of steam heated iron cylinders such that these cylinders heat the paper web sufficiently to cause evaporation of the remaining moisture.
In order to hold the web of paper firmly against the steam heated dryer drums, a dryer felt web may also be guided around the heated dryer drum overlaying the paper web. THe dryer felt web is maintained under tension so as to apply a uniform pressure against the paper web, thus improving the conduction of heat from the steam heated drum into the moisture bearing paper web. Since, under normal operating conditions, the dryer felt web is not intended to absorb water in liquid form, it is typically formed of a hard, generally non-absorbent fabric.
Other web drying techniques have also been used in the past to apply heat to the moving paper web so as to cause the moisture to evaporate from the web. In one technique, the web is passed beneath a series of gas burners which direct radiant infrared energy at the web. In another type of drying, the web is passed through a drying tunnel in which a plurality of air nozzles direct heated air against the web. This convection heating process is particularly useful in the final drying stages of the paper making process.
It has been found that the use of an electrostatic field may facilitate certain moisture removing techniques. U.S. Pat. No. 3,771,233, issued Nov. 13, 1973, to French et al, discloses a method of applying a high voltage direct current discharge to a liquid or a solid mass containing liquid, while the surface of the liquid or solid mass is in contact with a circulating gaseous atmosphere. Evaporation of the liquid is promoted by this technique due to turbulence of the atmosphere brought about by the discharge adjacent the surface of the liquid. The French et al disclosure is directed specifically to drying investment casting shell molds. The mold is placed in an oven for evaporation drying. The positive terminal of a high voltage d.c. power source is connected to the mold and to ground and a negative terminal of the power source is connected to a plurality of needlelike electrodes which surround but do not contact the mold.
Another approach to drying is disclosed in U.S. Pat. No. 2,740,756, issued Apr. 3, 1956, to Thomas in which a liquid bearing material, such as a paper web, is subjected to a high frequency pulsating uni-directional field. The field is said to drive the water out of the material web in liquid form without vaporizing the water. In the final drying phases, a high frequency bi-directional fluctuating field is preferred, however, for heating material having a relatively low percentage water content to cause evaporation.
A number of U.S. patents, issued to Robert R. Candor and James T. Candor, relate to the use of a static electrical field to assist in removal of water in a liquid form from various types of material, including paper, by causing the water to migrate physically in the direction of the field out of the moisture bearing material. These patents include U.S. Pat. Nos. 3,633,282, issued Jan. 11, 1972; 3,543,408, issued Dec. 1, 1970; 3,641,680, issued Feb. 15, 1972; 3,755,911, issued Sept. 4, 1973; 3,757,426, issued Sept. 11, 1973; 3,931,682, issued Jan. 13, 1976; 3,999,302, issued Dec. 28, 1976; and 3,977,937, issued Aug. 31, 1976.
The various embodiments disclosed in these patents relate to the removal of water from a moisture bearing web in liquid form. Although the Candor '282 patent discloses, in FIGS. 7 and 8, the use of a nonuniform electrostatic field in conjunction with a steam heated roll, each of the rolls has associated therewith a moisture absorbing felt web into which the moisture is driven, apparently in liquid form, by an electrostatic field produced between a plurality of small electrodes adjacent the drum and the grounded metal stem heated drum. Various other embodiments of the Candor invention are suggested but, as stated above, in each case the devices are intended to extract liquid water from the paper web without vaporization.
Additionally, the paper drying devices disclosed in the Candor patents are generally of the type which subject the paper web to a field by placing oppositely charged electrodes on opposite sides of the web or, in the case of the embodiment of FIG. 7 of the Candor '3,757,426 patent, by electrically connecting one side of a high potential source to the slurry forming the wet web and connecting the opposite side of the high potential source to a plurality of electrodes positioned beneath the web. It should be appreciated that an opposing electrode configuration may not be practicable in evaporation drying devices where heating apparatus must be positioned on one side of the paper web.
The Candor patents further suggest the use of suction, as in Candor '3,757,426, to assist in the removal of liquid water, as well as the use of vibrational energy or soundwaves, as in the Candor '3,931,682 and '3,641,680 patents, in conjunction with the use of an electrostatic field for removal of the liquid water. The Candor '3,999,302 patent further suggests dielectric heating in conjunction with electrostatic and vibratory liquid water removal, while the '3,977,937 patent suggests the use of patterned conductive belts for supporting the paper web and rearranging the position of the web fibers.
Removal of water in liquid form, however, is practicable only during the initial drying phases where the paper material still has a relatively high water content. For a paper web to be dried completely, however, it is necessary to supply heat in some manner to the paper web to evaporate the small remaining amounts of moisture. Evaporation is also the preferred drying mechanism where a web of material has been coated with a coating composition in liquid solution or suspension and it is desired to remove the liquid to produce a dry coated web. It will be appreciated that the known evaporation drying techniques require the application of substantial quantities of energy to the paper web and that, therefore, the drying efficiency of such techniques is extremely important. A number of Candor patents, such as U.S. Pat. Nos. 3,966,575; 4,081,342; 4,057,482; and 4,033,841, disclose drum dryers in which a plurality of electrode pairs, each pair including electrodes differing substantially in area, are provided on opposite sides of a web of moist paper. Half of the electrodes are positioned within the drum which must therefore be non-metallic, so as not to shield the electrodes.
Accordingly, it is seen that there is a need for a simple drying system and method for high efficiency evaporation drying of the type which is used for drying moisture bearing paper and coated paper material.